Cytoskeleton

Cytoskeleton: Structure, Function, and Movement

The cytoskeleton is the dynamic, internal scaffolding of the cell, composed of three primary protein filaments that govern cell shape, mobility, internal organization, and division.

1. Microfilaments (Actin)

Microfilaments are the smallest and most flexible components of the cytoskeleton.

Feature

Description

Structure

The smallest and most flexible. Forms a double helix of two F-actin polymers.

Monomer/Polymer

Monomer: G-actin (Globular actin). Polymer: F-actin (Filamentous actin).

Key Functions

Cell Migration/Diapedesis: Allows cells (like white blood cells) to change shape and squeeze through tight spaces. Cytokinesis: Forms the constriction/contractile ring that pinches one cell into two during cell division. Cellular Extensions: Forms the core of Microvilli (increases surface area for GI absorption) and Stereocilia (involved in hearing and balance). Muscle Contraction: Interacts with the motor protein Myosin to shorten muscle filaments.

2. Intermediate Filaments

Intermediate Filaments are the middle-sized and most resilient elements, providing tensile strength to the cell.

Feature

Description

Structure & Role

The middle size but the most tough and resilient filament. Resists compressive forces and helps maintain cell shape.

Cell Junctions

Forms the toughest cell-to-cell and cell-to-matrix anchors: Desmosomes (cell-to-cell) and Hemidesmosomes (anchors the cell to the basal lamina/extracellular matrix).

Tissue-Specific Types (Tumor Markers)

Lamins (Nuclear lamina), Keratin (Epithelial cells), Vimentin (Connective tissue/Fibroblasts), Desmin (Muscle cells), Neurofilaments (Neurons).

3. Microtubules (Tubulin)

Microtubules are the largest components, serving as the cell's rigid tracks for transport and movement.

Feature

Description

Structure

Largest filament. A hollow tube made of 13 protofilaments. Requires GTP for assembly.

Monomer/Polymer

Monomers: Alpha-tubulin and Beta-tubulin dimers.

Polarity

Minus end (points toward the nucleus) and Plus end (points toward the cell periphery).

Axonal Transport (The Cell's Railroad)

Microtubules provide the tracks for motor proteins to transport cargo across the cell:

Motor Protein

Transport Direction

Microtubule Movement

Cargo Example

Kinesin

Anterograde (Forward)

Minus end $\rightarrow$ Plus end

Neurotransmitter vesicles, newly synthesized proteins.

Dynein

Retrograde (Backward)

Plus end $\rightarrow$ Minus end

Old organelles, materials for degradation (e.g., lysosomes).

Role in Cell Motility (Cilia and Flagella)

Microtubules form the core structures responsible for cell movement:

Structure

Function

Flagella

Provides motility for the sperm cell to propel itself.

Cilia

Creates a propulsive beating action to move fluid or mucus (e.g., clearing the respiratory tract, moving the egg in the fallopian tubes).

Cilia and Flagella Microtubule Arrangement

Cilia and flagella have distinct arrangements of microtubules stabilized by a unique $9+2$ structure (called the Axoneme):

  • Basal Body (at the Base): $9 \times 3$ arrangement (nine triplets of microtubules), total 27 microtubules, with nothing in the center.

  • Axoneme (Shaft/Top): $9 \times 2 + 2$ arrangement (nine doublets surrounding two single microtubules in the center), total 20 microtubules.

Role in Cell Division (Mitosis)

Microtubules organize and separate chromosomes during mitosis:

  • Centriole: A structure composed of the $9 \times 3$ arrangement of microtubules.

  • Centrosome: Formed by two centrioles, it serves as the main microtubule-organizing center at opposite poles of the dividing cell.

  • Mitotic Spindles: Microtubules that extend from the centrosomes to connect to the chromosomes at the Kinetochore protein complex (located on the chromosome's centromere).

  • Chromosome Separation: The spindles pull apart the sister chromatids to opposite poles during Anaphase, ensuring each daughter cell receives a full set of genetic material.